I was told that I had a slight decrease in my bone density while on gnrh-a. Can you explain this?

Low estrogen. Estrogen is important for building bone and maintaining bone density. When one is on gnrha therapy (lupron) estrogen concentrations are low, and therefore bone is lost. Fortunately, after stopping therapy, bone density tends to return to normal.
Can happen. Gnrha are used to treat endometriosis or in medication protocols for assisted reproductive technologies. It works by disconnecting the communication between the brain and ovaries. The end result is very low estrogen levels, and if the gnrha is taken for less than 6 months the slight bone loss is usually reversible. Estrogen is needed to help the body absorb calcium to maintain good bone health.